Let’s play a word game! What’s the first word or phrase that comes to mind when I “say”:

“Vitamin C” --- citrus! 🍊

“Vitamin A” ---- carrots! 🥕🥕

“Vitamin D” ---- sunshine vitamin! ☀️

“Vitamin K” ---- Kale! Kale? Well, yes, kale is a great source of vitamin K. Good guess!  🥬

What is vitamin K?

Vitamin K is an umbrella name for a family of compounds that got its name from the German word for coagulation (Koagulation) in recognition of one of its main functions. Coagulation is the medical term for causing blood to become thicker and form a clot. Interestingly, the 1943 Nobel Prize in Physiology or Medicine was awarded to the two scientists who discovered vitamin K and its chemical structure. The two natural forms of this fat-soluble vitamin are known as K1 and K2.

· K1(phylloquinone) contributes 75-90% of all vitamin K in our diet and is mainly found in plant foods.

· K2(menaquinones) is a group of substances produced by gut bacteria which converts K1 into K2 and is only found in animal foods and fermented plant foods.

Vitamin K plays an important role in helping the body to stop bleeding. For example, when we cut ourselves, vitamin K is a critical element in the pathway that mobilizes clotting factors to halt the blood flow. In addition to this vital role, research is ongoing to identify vitamin K’s specific role in both bone health and heart health.

Where is vitamin K found?

The richest dietary sources for K1 are leafy green vegetables – like kale, spinach, collard greens, broccoli, and smaller amounts in other vegetables and fruits. Meat, dairy and some fermented foods provide the primary sources of K2. After vitamin K is absorbed by the body, it’s primarily taken up and stored in the liver.

How much vitamin K do we need?

The Institute of Medicine’s Food and Nutrition Board set an adequate intake (AI) of vitamin K for adults at 90 micrograms for women and 120 micrograms for men.  To give you an idea of how this intake would be met through food, here’s a list of several rich sources:


Micrograms (mcg) per Serving

Collard Greens, cooked, 1/2 cup


Brussels Sprouts, raw, 1 cup


Spinach, raw, 1 cup


Kale, raw, 1 cup


Broccoli, chopped, boiled, ½ cup


Vitamin K jumpstart

Because vitamin K stores do not transfer well from the mother to newborn babies, the American Academy of Pediatrics recommends that infants receive a single vitamin K injection at birth to prevent vitamin K deficiency bleeding (VKDB). Apart from newborns and people with malabsorption disorders, the US population has adequate vitamin K levels and deficiencies are very rare.

Who needs to be aware of vitamin K intake?

For some individuals who are at increased risk for developing harmful blood clots, an anticoagulant (blood thinner) like warfarin (Coumadin®) is prescribed. Warfarin acts by blocking an enzyme essential for activating the vitamin K available in the body. This rate-limiting action slows down the body’s ability to produce clotting factors. By disrupting this normal vitamin K-dependent process, the risk of blood clots is lessened. Not all blood thinners interact with vitamin K. But because warfarin’s direct action is focused on the vitamin K pathway, patients prescribed warfarin are counseled to maintain a consistent intake of vitamin-K rich greens to achieve the desired level of blood thinning (anticoagulant) effect.

A common misunderstanding is that leafy greens need to be avoided when warfarin is prescribed. In reality, the goal is akin to balancing a seesaw. By achieving a steady state of vitamin K intake, the correct dosage of warfarin is more effectively prescribed. For example, by including broccoli on one day, spinach the next, kale the next day, and so on, an individual can keep their vitamin K intake stable and enjoy the plentiful benefits of these nutrient-dense greens.

Ensuring the right balance

To maintain this important balance between vitamin K and warfarin, clinicians regularly request  a Prothrombin time, (PT) test be completed. Prothrombin is a protein made in the liver that is one of several factors involved in the clotting process. The PT test is done using a finger prick blood sample and measuring how fast it takes for blood to clot. Clinicians use the results of this test to ensure that the correct dosage of warfarin is prescribed.


Vitamin K may be an unsung hero when it comes to our body’s ability to support healthy blood clotting along with potential roles in bone and cardiovascular health.  Next time you enjoy a spinach or kale salad or side dish of broccoli, stop for a moment and appreciate the power in the bite of leafy greens. For those individuals prescribed warfarin, a steady supply of vitamin K-rich foods ensures the correct balancing act between both lifesaving medicine and beneficial leafy greens.